Thanks so much for all the pictures and tips. I happen to have a z400 at work I can salvage and this will save me a lot of guesswork!
Would it be at all possible to run just two 1080ti gpus installed inside the case on the mb (and 1 external)?
Two 1080Ti will not fit inside. Only short cards (no longer than the Gigabyte 1060 from my pics) will fit in the bottom slot.
Also the cards would essentially be touching one another due to thickness (if they would fit at all).
In summary -- only ONE big card can be used internally. Even two small cards isn't a terribly good idea for cooling purposes.